As a method of forming and solidifying a rapidly solidified aluminum alloy powder, there are various methods, for example, extrusion method, HIP method, powder forging method, etc. For solidifying a powder, it is necessary to heat the powder by all means, during which the quenching effect of the powder is lost to deteriorate the properties. Prevention of this deterioration is carried out by rapidly heating in a short time. A patented invention relating to carrying out rapidly heating for the purpose of solidifying a rapidly solidified aluminum alloy powder is 1. U.S. Pat. No. 4,435,213: "Method for Producing Aluminum Powder Alloy Products Having Improved Strength Properties; Alcore", a patented invention relating to induction heating of general powders, not limited to aluminum, is 2. U.S. Pat. No. 5,134,260: "Method and Apparatus for Inductively Heating Powders or Powder Compacts for Consolidation; Carnegie Melon University" and a patented invention relating to rapidly heating by hot air is 3. Japanese Patent Laid-Open Publication No. 158401/1991: "Method of Heating Rapidly Solidified Powders; Kubota".
To any one of the above described powder forging method, extrusion method and HIP method, well known in the art, is indispensable the heating before solidification because of 1. decreasing a deformation resistance of a powder and shaping the powder with a low stress and 2. degassing.
In particular, the degassing is an indispensable means for preventing a solidified article from forming of bubbles called blistering and in the case of the powder forging, strongly bonding grains with each other, for example, like known methods described in Japanese Patent Laid-Open Publication No. 224602/1987 and "Kei-Kinzoku (Light Metals)" 37 (10) 1987, page 656-664.
In the prior art techniques, degassing has generally been carried out by can-sealing a CIP (cold isotactic pressing) body and heating in vacuum or in an inert gas atmosphere at a temperature of 400.degree. to 600 .degree. C. Any prior art method has aimed at completing sufficiently degassing by raising the temperature for 0.5 to 2 hours and maintianing a predetermined temperature for 0.5 to 2 hours, amounting to 1 to 4 hours, using an ordinary resistance heating furnace.
However, it has hitherto been pointed out that the above described degassing method has the disadvantages that the quenching effect of a powder, i.e. the effect of precipitating finely and unifromly an element or phase which tends to be coarsely precipitated at an ordinary cooling rate or the effect of rendering crystal grains fine is lost by heating for a long time to deteriorate the properties of a shaped and solidified body, and moreover, prevention of oxidation during then needs controlling of the atmosphere, resulting in rising of the production cost.
Heating rapidly and uniformly a material having a low heat conductivity such as green compacts has generally been considered difficult. Ordinarily, the most suitable method for rapid heating on a commercial scale is an inductive heating. For example, it has been reported in Japanese Laid-Open Publication No. 134503/1974 that for powder metallurgy of a ferruginous metal, a high frequency induction heating is utilized for heating and sintering a green compact. Up to the present time, such a high frequency inductive heating has been utilized for previous heating in sintering in a shorttime or sintering forging (forging for increasing the density of a preform which is being sintered).
However, the inductive heating has not been used for degassing of a green compact of aluminum powder or aluminum alloy powder. The reasons therefor are as follows:
It has been considered that the presence of a stable alumina film (Al.sub.2 O.sub.3) with a low electric conductivity on surfaces of aluminum powder or aluminum alloy powder results in increase of the resistance of the powder and decrease of the electric conductivity of a green compact and an effective heating is impossible by induction heating since Joule's heat is hard to be generated in a material with a low electric conductivity, such as aluminum, eddy current is hard to be generated in the green compact and aluminum itself has a smaller magnetic permeability different from ferruginous materials.
Furthermore, it has been considered that even if the induction heating of the powder pressed compact can be carried out, a temperature difference between the surface and central part thereof is so large due to the low heat conductivity thereof that it is impossible to heat at a uniform temperature.
Under the situation, the present invention provides a method of degassing aluminum powder or an aluminum alloy powder comprising utilizing induction heating for a degassing means in a step of forming and solidifying the aluminum powder or aluminum alloy powder, whereby the above described disadvantages of the prior art can be overocme.
When solidifying a rapidly solidified aluminum alloy powder, the following subjects should be taken into consideration:
(A) A thermal history added to a powder is minimized so as to minimize deterioration of the texture of the powder by heating for solidification.
(B) Bonding of aluminum powder with each other is rendered as strong as possible.
(C) The solidification is carried out with a low cost.
Above all, for accomplishing the subject (A), the rapid heating method by the induction heating or hot air heating method as disclosed in the foregoing three patents is favourable. In the rapid heating method, however, there arises a problem that the bonding of aluminum powder with each other [subject (B)] is hard to take place. Therefore, the heating in the air, as described in Example of the patent 1, results in lowering of the fracture elongation even if the extrusion is carried out. For the purpose of compensating this subject, it is required to carry out the rapid heating in an inert gas, to carry out vacuum degassing before the solidification and to increase the elongation or fracture toughness value of the solidified material by extrusion or swageing working to cause a large plastic deformation after the solidification. In the patent 3, an apparatus for rapid heating in vacuum is provided. However, the subject (C), i.e. solidification with a low cost cannot be accomplished by addition of these steps.
The present invention provides means for solving the above described subjects (A), (B) and (C) and provides a solidified body having a higher strength and toughness without lowering mechanical properties as compared with any of solidifying methods of the prior art and a solidifying method for obtaining the same in economical manner.